Using a food processor, pulse together the flour and salt until combined. Add butter and pulse until mixture becomes coarse meal, about 15 pulses.
In a small bowl, combine egg and water. Add the egg mixture to food processor and pulse until it starts to clump together. DO NOT pulse until ball is formed. It should be crumbly with large clumps of dough.
Turn dough only a lightly floured surface and form into a ball. Cut in half and flatten each ball slightly to form a disk. Wrap each in plastic wrap and put in fridge for at least 1 hour or up to 3 days.
Preheat oven to 425°F and line a baking sheet with parchment paper. Dice sweet potatoes into ½ inch cubes and toss with 2 tablespoon olive oil. Lightly season with salt and pepper. Roast for about 20 minutes until tender and toss halfway through.
While sweet potatoes are roasting, heat a large skillet over medium heat. Add 1 tablespoon olive oil, onion and pepper and sauté for about 5 minutes. Add garlic and sauté for another 3 minutes. Mix in spices. Add drained black beans and corn and sauté for another 5 minutes. Remove from heat and add in the roasted sweet potatoes, cheese and lime juice. Mix well. Let cool to room temperature.
When ready to make empanadas, reduce oven to 375°F. Roll out dough to ⅛ inch thickness. Cut into circles about 5 inches in diameter using a ring cutter or small bowl.
Put about 2 tablespoons filling in the center of each circle. Put some water around the entire edge, so once folded it will stick together. Wrap dough around filling to form empanada, pressing edges together. Crimp with a fork. Place on a parchment-lined baking sheet.
Whisk together the egg and water to make the egg wash. Brush each empanada with the egg wash. Bake for 30-35 minutes or until they are golden brown. While empanadas are baking, mx all ingredients for the chimichurri together in a bowl. Allow to sit for 5-10 minutes to release all of the flavors into the oil before using.